Previous studies have shown that Merck & Co.’s Ebola vaccine can act fast and can maintain an antibody response for at least one year. Now, a new study has doubled that estimate.
The new study, published in Lancet Infectious Diseases, shows antibodies persist for two years after a single shot of Merck’s rVSV-ZEBOV vaccine.
Researchers examined participants from a previous phase 1 study who returned for follow-up research. All 44 of those patients who had received a high dose of the vaccine remained seropositive at two years, and 33 of 37 who got a lower dose were still seropositive.
Levels of glycoprotein-specific antibodies against the Zaire ebolavirus, the strain targeted by the shot, declined significantly from peak levels, but they still remained solid for both high-dose and low-dose groups, the study finds.
The research team also followed participants from two trials in Gabon and Kenya for a year, and found relatively similar stability in antibody response.
This means a single dose of the shot should be able to sustain antibody responses in different settings. That's important, the team noted, especially for resource-constrained regions where booster vaccinations would be impractical.
However, the fast-declining neutralizing antibody titer might be a problem for the vaccine’s clinical efficacy. In the Geneva study group, the proportion of participants who were seropositive with neutralizing antibody sharply dropped from 64% to 71% at 28 days to 27% to 31% at six months.
It is yet unknown which type of antibodies—neutralizing or glycoprotein-binding— might play a more important role in protection against Ebola. In a commentary that runs alongside the new Lancet study, two researchers suggest future studies on rVSV-ZEBOV track the titers of antibodies to other types of Ebola virus, “particularly since several conserved sites on the glycoprotein that are susceptible to neutralizing human antibodies are shared by all known species of Ebola virus.”
The Merck vaccine previously posted 100% efficacy in a “ring study” in Guinea, where researchers created a circle of vaccinations around known Ebola cases to stop the virus’ spread. Other studies in the U.S. and the West African country Liberia have shown the shot can elicit antibody responses that lasted at least for a year.
Merck has said it expected to file the vaccine for approval at a major regulatory agency in 2018. Though not yet approved by drug regulators, the WHO recommends that Merck’s shot be deployed under the Expanded Access framework should can Ebola outbreak occur.